The present invention relates in general to sensor circuits and, more particularly, to a circuit for correcting a nonlinearity in a sensor signal.
Sensors are commonly used for converting physical conditions such as temperature, pressure, and acceleration to an electrical sensor signal for further processing. A typical sensor, such as a pressure sensor, includes a diaphragm for converting a pressure into a stress. A transducer converts the stress into the sensor signal which is typically amplified and filtered to provide a sensor output signal.
Ideally, there is a linear relationship between the physical condition and the sensor signal. However, in most if not all sensors, the sensor signal does not accurately represent the physical condition because of nonlinearities introduced by the deflection of the diaphragm. In the case of a pressure sensor, an applied pressure induces a plurality of stress components on the diaphragm. For example, a bending stress that is linearly related to the deflection of the diaphragm produces a linear output signal which is representative of the applied pressure.
Another one of the plurality of stress components in the diaphragm is a membrane stress. The membrane stress is related to the thickness and physical dimensions of the diaphragm and is produced when the diaphragm is stretched. Membrane stress contributes an undesirable nonlinear component to the sensor signal which increases as the applied pressure increases. The nonlinear component gives rise to an error term such that the sensor output signal does not accurately represent the applied pressure. The magnitude of the nonlinear component can be as high as five or ten percent, and even higher with sensors designed for use in harsh environments.
Many applications, including fuel injection systems in automobiles, medical applications such as blood pressure instruments, and environmental control systems, require high sensitivity and accuracy of better than one percent. Prior art pressure sensors typically use physical structures such as bosses to reduce the error. The bosses are thick structures disposed in the diaphragm to increase rigidity and constrain the deformation of the diaphragm. However, bosses reduce sensitivity and are therefore not suitable for low pressure applications. Moreover, bosses increase both the die size and the complexity of the diaphragm, which increases the manufacturing cost of the sensor.
Hence, a need exits for a sensor having a substantially linear output signal that accurately represents the sensed physical condition.